1. Field of the Invention
This invention relates to an overload coupling device for thread-cutter chucks or quick-change inserts therefor. Such an overload coupling comprises a cylindrical drive sleeve loadable with the rotational drive moment, a drive shank drivable by means of the drive sleeve and held coaxially and rotatably within the latter, a mounting for the rotation-fast retention of a tap drill and a torque coupling seated in the force flux between drive sleeve and drive shank. The torque coupling has an adjustable tripping torque and is formed as a spring-loaded ball-pocket coupling with one or more coupling balls. The drive sleeve is axially non-displaceably held on the drive shank and has on one axial end face openings open in the axial direction, corresponding to the number of coupling balls and serving each for the reception of one coupling ball, the width of which openings is at least as great as the coupling ball diameter. The drive shank in the axial region of the openings comprises ball pockets corresponding to the number of coupling balls, arranged at preferably equal angular intervals from one another in the peripheral direction, depressed in spherical segment form or formed as bores, with diameter smaller than the diameter of the coupling balls, for the reception of the coupling balls. Such overload coupling devices are intended either for quick-change thread-cutter chucks, in which they are constructionally integrated, or for separate quick-change inserts which in turn can be received interchangeably in such quick-change thread-cutter chucks.
2. Description of the Prior Art
The overload coupling devices with torque coupling form safety couplings between the drive, which in the case of quick-change thread-cutter chucks inserted into a machine tool is provided on the machine and connected to the drive sleeve, and the drive output, that is the drive shank with tap drill held fast in rotation in its mounting. The force transmission from the drive input to the drive output takes place through the torque coupling. This has the task of protecting the driven tap drill against breakage, for example on running on to the bottom of a blind hole or when the tap drill has become blunt and thus higher torque absorption becomes necessary.
Thread-cutter chucks with torque-dependently working overload coupling devices are known in various forms, for example after the style of friction couplings, cam or dog couplings or even so-called ball-pocket couplings. Friction couplings have the disadvantage that on response the set torque drops off only slightly and the tap drill constantly remains under torque loading. Cam or dog couplings have the great disadvantage that on response they slip through from dog to dog, that is attempt suddenly and repeatedly to restore the drive connection, and separate the force flux again in the case of a counter moment lying above the tripping moment, so that the tap drills are subjected to alternating and suddenly acting torque loads, so that the cutting edges of the drills easily break or other drill damage can occur. Moreover considerable spring forces have to be applied, which is expensive and compels relatively large radial and axial dimensions. Various known ball-pocket couplings have substantially the same disadvantages, because in these the coupling balls frequently cannot yield sufficiently easily nor by a sufficient amount and suddenly repeatedly drop back into the position in which the drive connection is restored. Above all known ball-pocket couplings are subjected to considerable friction wear especially on account of the periodic re-coupling, especially if the smallest possible diameter of the chuck and/or insert is sought. Moreover the relatively complicated assembly of such ball-pocket couplings, which thus are very expensive, is disadvantageous. Their response behavior is also unsatisfactory, because inexact and unreliable, for the coupling balls frequently cannot yield with certainty or at least with sufficient certainty, so that there is no defined response threshold but a relatively wide response range.